As compared with widely used amorphous silicon (a-Si), amorphous (non-crystalline), oxide semiconductors have high carrier mobility, a wide optical band gap, and film formability at low temperatures, and therefore, have highly been expected to be applied for next generation displays, which are required to have large sizes, high resolution, and high-speed drives; resin substrates having low heat resistance; and others.
Among the oxide semiconductors, an amorphous oxide semiconductor consisting of indium (In), gallium (Ga), zinc (Zn) and oxygen (In—Ga—Zn—O, which may hereinafter be referred to as “IGZO”) has been preferably used in particular because of its extremely high carrier mobility. For example, Non-patent Literature Documents 1 and 2 disclose thin film transistors in which a thin film of oxide semiconductor having an In:Ga:Zn ratio equal to 1.1:1.1:0.9 (atomic % ratio) was used as a semiconductor layer (active layer). Patent Document 1 discloses an amorphous oxide containing elements such as In, Zn, Sn, Ga, and Mo, in which atomic ratio of Mo relative to total metal atomic contents in the amorphous oxide is 0.1 to 5 atomic %. Disclosed in an example is a TFT in which an active layer comprising Mo-added IGZO was used.
Properties of the oxide semiconductors are, however, known to vary depending on various deviations in the course of film formation process and subsequent heat treatment. For example, TFT characteristics are liable to deviate by significant change of carrier concentrations, a dominant factor of TFT characteristics, caused by lattice defects and hydrogen in the film generated in the course of the film formation process. It is thus essential from the point of view to improving the productivity to evaluate properties of deposited oxide semiconductor thin films, to feedback the results of the evaluation, to adjust manufacturing conditions, and to control film quality in the manufacturing process of the display devices or the like.
In a conventional evaluation method for an oxide semiconductor thin film, a gate insulator film or a passivation insulator film and contact electrodes are usually formed on the oxide semiconductor thin film, then the Hall effect is evaluated, and characteristics such as mobility and threshold voltage are measured. It takes, however, time and cost to form contact electrodes in such a contacting type evaluation method. Formation of the contact electrodes is also liable to induce additional defects in the oxide semiconductor thin film. It is thus required to establish a contactless-type evaluation method in which formation of contact electrodes is not necessary from the point of view to improving fabrication yield.
Therefore, microwave photo conductivity decay method, a method to quantitatively or qualitatively evaluate mobility of an oxide semiconductor thin film is disclosed as a method to control film quality of an oxide semiconductor thin film in a contactless manner without forming contact electrodes in Patent Document 2.
It is also required for a TFT adopting an oxide semiconductor thin film to be excellent in terms of not only the mobility but also resistance to stress (stress stability) such as light irradiation and voltage application. Here, the stress stability is defined that threshold voltage (Vth) shift is not observed, meaning a small amount of the threshold voltage shift (ΔVth) before and after applying the stresses, in drain current-gate voltage characteristics (I-V characteristics) even a semiconductor elements such as a transistor is subjected to continuous stresses such as holding light irradiation and continuously applying gate voltage.
In an organic EL display, for example, positive biasing is kept applying onto a gate electrode of a driving TFT while an organic EL element emits light. When a voltage is applied to the gate electrode, electric charges are trapped on the boundary between the gate insulator film and the semiconductor layer, resulting in a problem of variation of Vth and the switching characteristics. Such a variation of the switching characteristics of the TFT caused by the stress deteriorates the reliability of display devices such as a liquid crystal display and an organic EL display. Therefore, an improvement in the stress stability is eagerly desired.
The evaluation of the stress stability is also preferably conducted in a contactless and simple manner as for the mobility because the stability is dependent of film quality of an oxide semiconductor thin film. However, there has been a problem that actual measurement must have been conducted under applying the stress for an extended period of time after the formation of contact electrodes.